In conventional processes for preparing phthalic anhydride by oxidation of ortho-xylol or naphthalene in the vapor phase with an oxygen-containing gas, supported catalysts are employed. Such catalysts usually comprise, as the active portion, vanadium pentoxide and titanium dioxide, with minor additives such as stabilizers, promoters, etc., and as the support or carrier, particles of a non-reactive material, such as silicates or alumina, which supply the surface area, strength and stability required for the catalyst.
The catalysts for phthalic anhydride used in the past comprised supports or carriers preferably in the forms of balls, saddles, or cylinders.
For the active portion of the catalysts for phthalic anhydride, it has been common practice to use titanium dioxide with pores so distributed as to have pore diameters mostly in the 0.10 to 0.50 microns range.
While such known catalysts exhibit satisfactory catalytic activity in the preparation of phthalic anhydride, they are not entirely devoid of shortcomings, as connected with their potency and selectivity levels.